Carburization is the conventional process for the case hardening of steel. In gas carburizing the steel is exposed to an atmosphere which contains components capable of transferring carbon to the surface of the metal from which it diffuses into the body of the part. In many carburizing processes, an important constituent of the furnace atmosphere used to carburize metal parts is the carrier gas. A carrier gas serves to provide a furnace atmosphere with a positive carbon potential.
A variety of carrier gases have been employed in carburizing as discussed in U.S. Pat. No. 4,049,472, but the most common carrier gas is the endothermic (endo) gas derived by partial combustion of natural gas in air. When using endothermic gas, it is usually necessary to add a relatively small quantity of another constituent (i.e., enriching gas), usually natural gas, to the atmosphere to raise the carbon potential of the furnace atmosphere.
A thorough discussion of carburizing can be found in the section entitled "Furnace Atmospheres and Carbon Control" found at pages 67 through 92, and that portion of the section entitled "Case Hardening of Steel" appearing at pages 93 through 128 of Volume 2 of the Metals Handbook published in 1964 by the American Society of Metals, Metals Park, Ohio. This particular volume of the Metals Handbook is entitled "Heat Treating, Cleaning and Finishing."
During gas carburization, the steel objects to be carburized are exposed at an elevated temperature, usually in the range of about 1400-1700.degree. F., until carbon penetration to a desired depth has been achieved. The steel objects can then be cooled to room temperature by various known methods such as a furnace, air or media quench to develop the desired physical properties and case hardness in the finished article.
The basic endothermic atmosphere produced by the incomplete combustion of natural gas in air consists of approximately 40% N.sub.2, 40% H.sub.2 and 20% CO. The reaction by which carbon is generally believed to be deposited on the surface of the steel is represented by the following equation (1): EQU H.sub.2 +CO=C+H.sub.2 O (1)
The water produced in equation (1) immediately reacts partially with more CO according to the well-known water gas shift reaction (2): EQU H.sub.2 O+CO=CO.sub.2 +H.sub.2 (2)
Equations (1) and (2) may be added together to yield reaction (3): EQU 2CO=C+CO.sub.2 (3)
The net results of carburization by the endothermic atmosphere is the decomposition of carbon on the surface of the metal and concurrent formation of an equivalent amount of CO.sub.2 or H.sub.2 O. These two substances, CO.sub.2 and H.sub.2 O, cause the reversal of reactions (1) and (3) and if allowed to accumulate, would quickly bring the carburization process to a halt. The purpose of adding enriching gas as mentioned above is to remove the H.sub.2 O and CO.sub.2 and regenerate more active reactive gases according to reactions (4) and (5): EQU CO.sub.2 +CH.sub.4 =2CO+2H.sub.2 (4) EQU H.sub.2 O+CH.sub.4 =3H.sub.2 +CO (5)
Another method of generating a carburizing atmosphere which is discussed in U.S. Pat. No. 4,306,918 involves decomposition of methanol, either alone or in combination with nitrogen, according to equation (6): EQU CH.sub.3 OH=2H.sub.2 +CO (6)
It will be noted that the above reaction (6) results in the production of CO the same as is produced by the use of endothermic atmosphere by partial combustion of natural gas. By use of the appropriate quantities of nitrogen and methanol it is possible to generate a synthetic atmosphere which is essentially identical in composition to that produced by the partial combustion of natural gas.
During operation of a furnace, the carbon potential of the furnace atmosphere can become too high. When the carbon potential becomes too high it can lead to the excessive formation of carbon on the metal parts being carburized and the formation of excess carbon in the interior of the furnace. In order to lower the carbon potential of a furnace atmosphere, it is a common practice to acid air to the process chamber of the furnace. Air has also been employed in the prior art in situations where a carrier gas is not generated outside the process chamber. Specifically, air has been used in situations where a combustible gas (e.g., natural gas) is fed into a generator located within the process chamber of the furnace. In such situations, the air serves to decompose the combustible gas in situ in the process chamber of the furnace. U.S. Pat. No. 3,290,030 discloses an apparatus for the generation of furnace atmosphere wherein air and gaseous hydrocarbons are fed into an apparatus which is either located in or beneath the process chamber of the furnace. The air and gaseous hydrocarbons combust in the apparatus to form the furnace atmosphere.
The present invention provides a new and improved apparatus for generating furnace atmosphere in-situ in the process chamber of the furnace using air and an enriching gas. Applicant's apparatus obviates the need for a separately generated carrier gas.